Manacher_longest_palindromic_prefix_or_suffix.cpp

// Problem : https://codeforces.com/problemset/problem/1326/D2

#include <bits/stdc++.h>
#define int long long
#define sz(x) (int)(x.size())
using namespace std;

const int N = 1e5 + 1, M = 5e3 + 5, inf = 1e18, mod = 1e9 + 7;

string Manacher(string s) {
  int n = sz(s);

  vector<int> d1(n);
  for (int i = 0, l = 0, r = -1; i < n; i++) {
    int k = (i > r) ? 1 : min(d1[l + r - i], r - i + 1);
    while (0 <= i - k && i + k < n && s[i - k] == s[i + k]) {
      k++;
    }
    d1[i] = k--;
    if (i + k > r) {
      l = i - k;
      r = i + k;
    }
  }

  vector<int> d2(n);
  for (int i = 0, l = 0, r = -1; i < n; i++) {
    int k = (i > r) ? 0 : min(d2[l + r - i + 1], r - i + 1);
    while (0 <= i - k - 1 && i + k < n && s[i - k - 1] == s[i + k]) {
      k++;
    }
    d2[i] = k--;
    if (i + k > r) {
      l = i - k - 1;
      r = i + k;
    }
  }

  int maxLen = 1;
  for (int i = 0; i < n; i++) {
    if (d1[i] == i + 1)
      maxLen = max(maxLen, 2 * i + 1);
    if (d2[i] == i)
      maxLen = max(maxLen, 2 * i);
  }
  return s.substr(0, maxLen);
}

string findBest(int l, int r, string S) {
  string s = S.substr(l, r - l + 1);
  string bestPref = Manacher(s);
  reverse(s.begin(), s.end());
  string bestSuff = Manacher(s);
  return (sz(bestPref) > sz(bestSuff) ? bestPref : bestSuff);
}

void solve() {
  string s;
  cin >> s;
  int n = sz(s);
  int l = 0, r = n - 1;
  while (l < r && s[l] == s[r])
    l++, r--;
  if (l >= r) {
    cout << s << '\n';
    return;
  }
  string ans = s.substr(0, l) + findBest(l, r, s) + s.substr(r + 1);
  cout << ans << '\n';
}

int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);

  int t;
  cin >> t;
  while (t--)
    solve();
}